1. Field of the Invention
The present disclosure relates generally to metallic glass alloys, more particularly to metallic glass alloys that include six or more elements.
2. Related Art
Metallic glass alloys (MGAs), or metallic glasses, are amorphous metals and have been reported as existing in thin ribbon form since as early as the 1950s. Metallic glasses differ from conventional metals in that they lack crystalline structure. The atoms in the amorphous structure are randomly arranged, like in a liquid, rather than sitting on a repeatable, orderly lattice. This lack of crystalline structure means that metallic glasses also lack crystalline defects, such as grain boundaries and dislocations. Without these defects metallic glasses exhibit extraordinary mechanical properties, magnetic behavior, and corrosion resistance.
Because the equilibrium structure for a metal alloy is typically crystalline, amorphous metals can only be produced by rapid cooling from the liquid state. Until recently, the cooling rates required were on the order of 105-106 K/s, which limit the thickness of a fully amorphous alloy to fractions of a millimeter. The resulting ribbons and wires are used extensively as transformer cores and magnetic sensors, but the small dimensions limit the structural applications of the material.
The recent development of bulk metallic glasses has opened the door for use of these fascinating materials in structural applications. These alloys require cooling rates of only 1-100 K/s, so fully amorphous castings up to a centimeter thick can be manufactured using conventional casting methods. Metallic glass alloys are used in golf clubs, fishing rods, car bumpers, aircraft skins, artificial joints, dies, armor-piercing projectiles, engine parts, and cutting tools.
The recent development of zirconium (Zr)-based MGAs (compositions with much lower critical cooling rates and thus castable in thicker sections) are interesting candidates for structural material applications because of the increased thickness (Johnson, W. L., “Bulk Glass-Forming Metallic Alloys: Science and Technology,” MRS Bulletin, 24(10):42-56, 1999). Specifically, these MGAs generally possess very high elastic strain limits (2 to 3%) and therefore very high yield strengths (about 1.6 GPa). Beyond their elastic limits, however, MGAs do not strain harden, and plastic deformation is immediately localized into shear bands. Shear bands thus serve as a MGA's sole mechanism of plastic flow, under quasi-static as well as dynamic stress loads. The localization is generally modeled as resulting from a reduction in local viscosity, associated with an increase in “free volume” as atoms move within the amorphous structure, but there is not a universally agreed-upon explanation for this behavior (Spaepen, F., “A Microscopic Mechanism for Steady State Inhomogeneous Flow in Metallic Glasses,” Acta. Met., 25(4):407-415, 1977). At higher strain rates, the additional thermal-softening component leads to an earlier failure along one of the first shear bands, reducing the net accumulation of the plastic deformation (Subhash, G., R. J. Dowding, and L. J. Kecskes, “Characterization of Uniaxial Compressive Response of Bulk Amorphous Zr—Ti—Cu—Ni—Be Alloy,” Mat. Sci. and Eng., A334(1):33-40, 2002).
Unfortunately, most Zr-based MGAs have relatively low densities of less than 7 g/cm3. Coupled with typical failure strengths of 1.6 GPa, their use disallows compressive load-bearing applications which require higher densities and higher strengths, and without the customary plastic flow and deformation.
Current quinary hafnium (HO-based MGAs, with higher densities of at least 10.5 g/cm3 still remain relatively poor glass formers. That is, their glass forming ability, measured by Trg, the reduced glass transition temperature, defined as the ratio of the glass transition temperature to that of the liquidus temperature, ranges from about 0.58 to 0.62. However, typically, good glass forming MGAs will have Trg values of 0.63 to 0.67.
Therefore, it would be desirable to alter MGA chemistry to improve upon Trg.
Thus, there is a need for new metallic glass alloys.
There is still another need for metallic alloys that made from or otherwise include six or more elements.
Yet another need exists for glass metallic alloys having a density greater than 7 g/cm3.